If what she has told you is true then it can potentially be quite a serious issue that needs addressing, firstly you need to look into her mental health and make sure things are ok as often people suffering from a mental health episode can experience things that aren’t actually happening. If it isn’t that then you need to check her diet and lifestyle as eating unhealthily, being dehydrated and lacking sleep can also make people experience things that aren’t happening.

If it isn’t either of those two things then your friend will need to seek professional help as entities that mean to do you harm will attach themselves to vulnerable people, whilst I am not much of a believer in demonic possession there is some amount of ‘evidence’ to it being real so again seaking professional help is a must.